The present invention relates to a projection system in which images which are formed in a matrix type light valve are irradiated by an illumination light beam to be projected onto a screen through a projection lens.
To obtain large pictures on a screen, a conventional method exists whereby the light beam from a light source is irradiated through a small light valve which forms optical images according to picture signals and the optical images are enlarged and projected onto the screen. In such a projection system, the resolution of projected images is almost determined by the resolution of the light valve itself, and the light output increases as the light flux of the light source is increased. Therefore, a projection system of a high resolution and a strong light output can be realized with a light valve of a high resolution and a strong light source, even if the image surface of the light valve is small. On the other hand, in recent years, a system using a liquid crystal as a light valve has been highlighted. For instance, in "LCD Full-Color Video Projector", SID 86, Digest, page 375, Morozumi et al. have proposed a projection system using a liquid crystal light valve of the active matrix type as the light valve (hereinafter called simply "light valve").
FIG. 7 shows an example of the fundamental construction of a conventional projection display system using a liquid crystal as a light valve. A light beam which is emitted from a light source 1 is transformed into an approximately parallel light beam by a converging optical apparatus 2 and is transmitted through a light valve 3 and enters a projection lens 4. The light valve 3 is constructed by arranging a liquid crystal cell 6 and polarizing plates 7 and 8 on both sides of the liquid crystal cell 6 to form a crossed Nicols or a parallel Nicols. Optical images corresponding to the picture signals are formed as spatial changes of light transmittance in the light valve 3, and the optical images are enlarged and projected onto a screen 5 through the projection lens 4. The optical system consisting of the elements from the light source 1 to the projection lens 4 is referred to as a projector 9.
FIG. 8 shows an example of a rear type projection system which is constructed using the projector 9. The light beam emitted from the projector 9 is transmitted via a first plane mirror 10 and a second plane mirror 11, being enlarged and projected onto the transmission type screen 5.
With the construction shown in FIG. 8, the depth of the projection system is determined by the depth of the projector 9 and the projection distance of the projection lens. Therefore, to realize a compact projection system, it is necessary to reduce the depth of the projector and to widen the field angle of the projection lens. However, the following problems must be solved relating to the projection lens.
(1) To project the optical images which are formed in the light valve in accordance with the picture signals onto the screen with a high contrast, it is necessary to make use of light rays which are emitted from the light valve plane at an angle near the right angle. Therefore, the projection lens needs to be constructed so that the principal rays of the projection lens are set to an angle within 15.degree. from a normal of the liquid crystal light valve plane.
(2) Since matrix electrodes are used in the liquid crystal light valve, different from the CRT projection system, it is difficult to electrically correct figure distortions of the projected images. Therefore, it is necessary to set a distortion aberration of the projection lens to a value within .+-.1%.
The above-mentioned problems generally make it difficult to widen the field angle of the projection lens, and are not limited to the liquid crystal but are also common in a light valve employing the birefringence or optical rotary power of an electro-optic crystal, a PLZT, or the like, in which images are formed as spatial changes of light transmittance according to an electric signals.